Competitive interplay of fracture number and aperture in thermo-hydro-mechanical coupled heat extraction from enhanced geothermal systems

Increasing fracture connectivity and hydraulic conductance can improve fluid circulation in enhanced geothermal systems, while faster reservoir cooling can offset the resulting gains in heat output. This study examines how natural-fracture number and aperture jointly affect this trade-off over time using a two-dimensional coupled thermo-hydro-mechanical model. Thirty-six simulations combine three fracture-network realizations, natural-fracture numbers of 10, 20, and 40, and apertures of 0.10–0.22 mm over 50 years of constant-pressure operation. Fracture apertures remain fixed during each simulation, while matrix porosity and permeability evolve through the adopted constitutive relations. Larger fracture numbers and apertures generally increase mass flow and cumulative gross sensible-heat extraction, but reduce production temperature and advance thermal breakthrough. A 10% loss of the initial reservoir-to-injection temperature difference occurs within 50 years in 23 of the 36 cases. Quadratic response-surface sensitivities show a shift in thermal-power control from aperture at early times to fracture number at later times, with the first crossover occurring between years 17 and 21 across the three realizations. Local matrix permeability also partially recovers during continued cooling in one examined case, without establishing recovery of reservoir-scale thermal performance. Requiring the year-30 production temperature to exceed 150 °C in all three realizations retains nine of the 12 tested number–aperture combinations. These results show that parameter rankings depend on both the performance metric and the operating horizon. Joint assessment of flow, temperature, and cumulative heat provides a basis for temperature-constrained comparison of the tested configurations; economic ranking additionally requires pumping energy and cost estimates.